Exogenous application of proline alleviates B-deficiency-induced injury while aggravates aluminum toxicity in trifoliate orange seedlings

Abstract

Boron (B)-deficiency and aluminum (Al) toxicity are two major factors limiting plant growth. The objective of this study was to investigate the effects of exogenous proline (Pro) on the antioxidant enzyme system and leaf structure under B-deficiency and Al toxicity. Citrus seedlings were exposed to 10 μM B (CK), 0.1 μM B (–B), 300 μM AlCl3·6H2O (+Al), and double stress with 0.1 μM B and 300 μM Al (–B + Al), while 0 and 0.2 mM proline levels were applied to the nutrient solution. The results showed that exogenous proline limited plant growth, similarly, both B-deficiency and Al toxicity showed extremely inhibition on seedlings and exhibited higher oxidative stress compared with the control. Addition of proline attenuated B-deficiency-induced H2O2 accumulation in leaves by regulating antioxidant enzyme system in terms of membrane permeability, malondialdehyde (MDA). Moreover, Fourier transform infrared spectroscopy (FTIR) revealed that exogenous proline increased cellulose and protein contents under B-deficiency. By contrast, proline supply considerably inhibited plant growth-related parameters accompanied by higher MDA and H2O2 contents under Al stress. Taken together, our results suggest that exogenous proline is deleterious to seedling growth. Exogenous proline alleviated B-deficiency-induced injury on plants compared to without proline under B-deficiency, while aggravated Al toxicity. This study may provide physiological insights into understanding the mechanisms of B-deficiency in plants with exogenous proline.